1. Field of the Invention
The present invention relates generally to an f.theta. lens for use in a light beam scanner and more particularly to an f.theta. lens composed of two positive-meniscus lenses for use in a light beam scanner.
2. Discussion of the Background
Light beam scanners for deflecting a monochromatic light beam such as a laser beam or a quasimonochromatic light beam such as a light beam emitted from a monochromatic fluorescent lamp have been widely known in relation to optical printers and information readout devices.
In such light beam scanners, the light beam is converged into a light spot on a surface being scanned such as the surface of a photosensitive recording medium or an original document carrying information to be read out. Since the light beam is normally deflected by a rotating polygonal mirror, a hologram grating disc, or the like, the light beam is deflected at a constant angular velocity. Where the deflected light beam converged by an ordinary convergent lens on the surface being scanned, the focused beam spot does not move on the surface or scan the surface at a uniform speed.
An f.theta. lens is a lens used for equalizing the scanning speed of a light beam which is being deflected at a constant angular velocity. When a light beam to be deflected falls on an f.theta. lens at an incident angle of .theta., the distance of a focused beam spot on the surface being scanned from the optical axis is given by f.theta. where f is the focal length of the f.theta. lens.
Single-element f.theta. lenses among conventional compact f.theta. lenses require a substantially long optical path to obtain a desired scanning area length since the angle of deflection that can be covered by a light beam deflected by the f.theta. lens is small. Therefore, a light beam scanner employing such f.theta. lens cannot be of a compact construction.
A prior two-element f.theta. lens designed for light beam deflection through a wide angle is disclosed by Japanese Laid-Open Patent Publication No. 59(1984)-147316. The effective deflection angle, or the allowable range of a deflected light beam with respect to its incident angle, of the disclosed f.theta. lens is relatively large, i.e., 70 degrees or greater. The f.theta. lens however requires as one element a special rotationally asymmetric lens known as a toric lens, and hence cannot easily be manufactured and is costly to manufacture.